The present invention relates generally to fluorescent lamps and more particularly to high color rendering fluorescent lamps.
Color rendition is a measure of the light reflected by a color sample under a given light source, compared to the light reflected by the same sample under a standard light source. Color rendition is calculated as disclosed in xe2x80x9cMethod of Measuring and Specifying Colour Rendering Properties of Light Sources, 2nd Editionxe2x80x9d, International Commission on Illumination, Publication CIE No. 13.2 (TC-3.2) 1974, the contents of which are hereby incorporated by reference. The differences in value, chroma and hue of the light reflected under the two sources are measured and summed, the square root of the sum is taken, multiplied by a constant, and subtracted from 100. This calculation is done for 14 different color standards. The color rendering index for each of these standards is designated Ri. The General Color Rendering Index, Ra, is defined as the average of the first eight indices, R1-R5. The constant has been chosen such that Ra for a standard warm white fluorescent tube is approximately 50. It should be noted that an Ra of 100 corresponds to a light source under which the color samples appear exactly as they would under a standard light source, such as an incandescent (black body) lamp or natural daylight.
In certain commercial and residential applications very high color rendition is desirable. Examples include cinema productions, grocery and clothing stores, photographic studios, areas where color comparisons are being made, museums, etc. Although standard fluorescent lamps have many advantages, such as providing diffuse uniform lighting, relatively high efficiency, and low heat generation, they are often inadequate for these applications, as they typically have color rendition indices of 50-85.
Some high color rendering phosphor blends have been developed for these applications. In the tri-phosphor systems used in conventional fluorescent lamps, the phosphors are typically chosen in order to provide three peak emissions, one red, one blue, and one green. The mixture of these three emissions generates the generally white light emitted from the lamp. To produce high color rendering phosphor blends, the phosphors are chosen in order to xe2x80x9cfill inxe2x80x9d the visible spectrum, i.e. provide emission at substantially all wavelengths across the visible spectrum. U.S. Pat. Nos. 3,778,660, 4,296,353, 4,602,188, 4,644,223, 4,705,986, 4,527,087, 4,891,550 and 5,350,971, the contents of which are incorporated herein by reference, all suggest various phosphor blends for increasing the color rendering properties of fluorescent lamps.
Specifically, U.S. Pat. No. 4,705,986 to Iwama et al. (xe2x80x9cthe ""986 patentxe2x80x9d) discloses phosphor blends that yield color rendering indices of 98-99 at 5000 K correlated color temperature (CCT). However, in order to achieve such high indices with the blends disclosed in the ""986 patent, it is necessary to utilize two separate phosphor layers.
U.S. Pat. No. 3,778,660 to Kamiya et al. (xe2x80x9cthe ""660 patentxe2x80x9d) discloses phosphor blends that yield color rendering indices as high as 97, but cannot achieve color rendering indices higher than 97.
Also, it is difficult to get very high color rendering of saturated reds as measured by the color rendering index R9. U.S. Pat. No. 4,527,087 to Taya et al. discloses phosphor blends which achieve a value of Ra of 99 at 5200 K (CCT). However, the blends disclosed in that reference cannot achieve a value for R9 greater than 97. High color rendition of certain other colors, such as vegetable green, flesh tones, etc. is also generally not achieved.
Also, the above patents disclose phosphor systems which achieve high color rendition for lamps with CCTs of greater than 5000 K. In North America and Europe, people often prefer lower color temperature lamps. The most popular fluorescent lamps are cool white (CCT=4100 K), white (CCT=3500 K) and warm white (CCT=3000 K). It is more difficult to achieve very high color rendition values at the lower color temperatures for which the reference sources are incandescent radiators rather than daylight.
Finally, some of the high color rendering phosphor blends which are on the market utilize 5 or 6 or more different phosphors. Blending such a large number of phosphors to hit a desired color and spectrum is difficult, and this needs to be done repeatedly because the properties of the phosphors may change from lot to lot.
There is a need to achieve higher color rendering than has been heretofore possible in fluorescent lamps. The fluorescent lamps of the present invention render all the CIE muted colors and all special colors so that they are virtually indistinguishable from their appearance under an incandescent or daylight source. The present invention provides lamps with color temperatures from 2700 K or 2900 K to 6500 K or 6600 K which achieve Ra values of 98-99. All special color rendition indices are greater than 90, and in particular, the saturated red color rendition index, R9 is greater than 97.
There is a further need to achieve these very high values of the color rendition indices with a minimal number, i.e. 3 to 4, phosphors in a blend.
There is a further need to achieve the very high color rendition by employing a filter to absorb radiation between 400 nm and 450 nm and thereby reduce the intensity of the mercury lines at 405 nm and 435 nm.
There is a further need to achieve the desired high color rendition by blending the phosphors in precise ratios thus producing a balanced spectrum. The amount of each phosphor is preferably adjusted so that the color rendition index is a maximum.
A mercury vapor discharge lamp is provided which comprises a glass envelope, means for providing a discharge, a discharge-sustaining fill of mercury and an inert gas sealed inside the envelope, and a phosphor-containing layer coated inside said glass envelope. The phosphor blend in the phosphor-containing layer is 40 to 80 weight percent of a first phosphor having an emission band with a maximum between 610 nm and 640 nm and having a half-value width of 10 nm to 100 nm, 0 to 20 weight percent of a second phosphor having an emission band with a maximum between 620 nm and 660 nm and having a half-value width of 1 nm to 30 nm, 8 to 50 weight percent of a third phosphor having an emission band with a maximum between 460 nm and 515 nm and having a half-value width of 50 nm to 120 nm, and 0 to 10 weight percent of a fourth phosphor having an emission band with a maximum between 530 nm and 560 nm and having a half-value width of 2 nm to 130 nm.